The present invention relates to the concentration and collection of radiant energy and more particularly a means for concentrating solar radiation without tracking the daily and seasonal motions of the sun.
Many devices have been developed to concentrate the radiation from the sun primarily to obtain elevated temperatures by converting the focused optical solar energy into heat. At higher temperatures the energy is more useful for heating dwellings, for doing useful work such as fueling steam turbines, and for the generation of electricity with pyroelectric devices. Large lenses and reflecting mirrors can be moved to continually point toward the sun and such tracking concentrators can attain very large concentration ratios and very high temperatures in a small focal region. However, the complication of moving and pointing mirrors or lenses have rendered tracking concentrators economically impractical for domestic heating and electrical power generation at homes or at other consumer locations.
Techniques have been advanced to concentrate the sun's optical flux without motion or tracking. Trough shaped reflectors which are oriented along East-West lines have been developed (U.S. Pat. No. 4,002,499) with acceptance angles in the elevation plane of 20.degree. to 35.degree.. These compound parabolic concentrators have large side walls making them structurally deep and they do not provide the acceptance angles in the elevation plane of approximately 50.degree. needed to accomodate the annual angular variations of the sun.
Other mirrors and lenses with wide acceptance angles are available, such as convex circular cylinders and Fresnel lenses. However, their focal surfaces, where blackened tubes are located to absorb the radiant energy, are so large in order to intercept the incident sun's energy from all directions within the acceptance angles that thermal radiation from these large structures causes severe heat loss, loss of efficiency and reductions in temperatures in the focal zone. The image surface area to aperture area must be reduced to provide a high concentration ratio and high operating temperature. This task is particularly difficult when large acceptance angles are required.
Efforts have been made to obtain high concentration ratios for non-tracking concentrators with low to moderate acceptance angles. The employment of secondary mirrors with circular, elliptical and hyperbolic shapes has been done (U.S. Pat. Nos. 3,125,091 and 3,868,823) to improve the focusing of optical concentrators for a given direction of the sun, but are ineffective for a plurality of directions and, hence, do not significantly improve the acceptance angle of the concentrator. A method for switching in only the portions of the focal region directly heated by the sun by means of thermostatic controls (U.S. Pat. No. 3,915,148) has been devised for use with wide angle Fresnel lenses. This apparatus, in addition to the complexity of switches and thermostats, requires tubing and other heat radiating structures over the focal region adding the thermal inertia, increasing radiation losses, and making it difficult to insulate adjacent receiving tubes.
Thus, there exists a need for a new non-tracking non-switching solar concentrator which will shrink or collapse the optical focal region of a wide acceptance angle collector in order to reduce the size and number of tubes or electrical conversion surfaces while, at the same time, obtaining high concentration ratios, operating temperatures and efficiencies.